The invention relates to processes and applicators for applying waves to a material, in particular to a confined body of material at least one of whose main components is a dielectric, particularly in the solid or liquid phase, the waves being electromagnetic microwaves with frequencies between 1 MHz and 40 GHz, or better still between 500 MHz and 4 GHz.
An applicator consists essentially of a waveguide for microwaves, whose inlet is coupled to a generator as a source of these waves. The waveguide also serves as a receptacle or tunnel for the material being treated, or the waveguide can terminate in a horn, which contains the material. In the present context the inlet face of the waveguide means the face of the material, or the face of the receptacle which contains the material, if the material is a fluid or a dispersion. This is the face through which the incident waves from the generator penetrate into the material. The outlet face of the waveguide is the face of the material through which the wave emerges. In those cases where the inlet face is not perfectly defined geometrically, the face is the plane, perpendicular to the direction of propagation of the wave, at which the energy conveyed by the incident wave is reflected or modified due to the presence of the material or its associated devices. This definition for the inlet face allows one to define unambiguously the complex coefficient of reflection at the inlet of the waveguide, for the mode of propagation, or modes of propagation, in question, and by deduction the inlet and outlet impedances.
Applicators of this kind are used in a variety of technical fields, ranging from the processing of foods, the heating of insulating materials, the drying of adhesives, the setting of concretes and the formation of polymers to desorbtions and other physical and chemical processes.
The yield of a process of this kind, that is to say the ratio of the energy consumed in the waveguide to the energy in the incident wave, is greater the less the wave is reflected from the inlet face. Reflection from the inlet face also has other disadvantages, in that a system of stationary waves is formed which impairs the good functioning of the generator. Furthermore it is often necessary to prevent reflection from the inlet face for the safety of the personnel working in the neighbourhood of the installation.
An apparatus described in the U.S. Pat. No. 3,712,971 comprises two waveguides coupled to a generator and equipped with slots through which the material being treated can pass. Means are provided for preventing the reflected microwave energy from reaching the generator. But a wave passing through one of the waveguides can return through the other one to the generator without having been reflected, damaging the generator. Furthermore controls of the apparatus depends on the humidity of the material being treated, which is always changing.
The invention gets over these difficulties by preventing the wave which has passed through one of the waveguides from returning through the other, even if the apparatus is not correctly adjusted.